This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2000-259443, filed Aug. 29, 2000, the entire contents of this application is incorporated herein by reference.
This invention relates to a detector for obtaining two-dimensional radiation image using a scintillator and a phosphor as radiation detecting medium. The detector is characterized in that by combining a scintaillator and phosphor of short fluorescence life with wavelength shifting fibers, two-dimensional radiation image can be detected at high speed and with good positional precision even if the incident radiation has high flux. If combined with a neutron converter, the detector is also capable of two-dimensional neutron imaging and hence is used for studies in materials physics and structural biology by neutron scattering in nuclear reactors and accelerators or X-ray scattering with synchrotron radiation, for medical X-ray diagnosis using X-ray generator and accelerator, and for autoradiography using X-rays or neutrons. The detector is also used in understanding dynamic events through fast processed and real-time radiation image detection with a radiation image detector for studies in high-energy physics using an accelerator, as well as in a function apparatus for monitoring the distribution of radiations including neutrons generated in nuclear reactors and fusion reactors.
The two-dimensional radiation image detector has been used to determine the position of entrance of a radiation into a scintillator or a phosphor by detecting the emitted fluorescence with bundles of optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction. In the case of a detector using scintillators, one pixel need be formed by one scintillator as shown in FIG. 31 [K. Kuroda et al., Nucl. Instr. and Meth. A430 (1999) 311-320] or FIG. 32 (Masaki Katagiri, JPA 2000-187077), so scintillators of a comparatively large size have been used to construct a two-dimensional radiation image detector that has a positional resolution of at least about 5 mm and which has a comparatively large area.
In the case of phosphors, a thin fluorescent sheet is used and detection is realized by bundles of optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction, so in order to enhance the detection efficiency of fluorescence, the distance between adjacent bundles of optical fibers has to be shortened; hence, the use of phosphors has been limited to a two-dimensional radiation image detector that has a positional resolution of no more than about 2 mm and which has a comparatively small area.
In either type of detector, scintillators or phosphors having short fluorescence life are used and detection is realized by using optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction, so except for some phosphors such as ZnS:Ag that emit a large amount of fluorescence, no more than several tens of photons reach the photodetector. Therefore, utilizing the emission of fluorescence that occurs upon incidence of radiation according to the Poisson distribution, K. Kuroda et al. have proposed a method by which the fluorescence from horizontal and vertical wavelength shifting fibers is converted to a two-dimensional image by a signal processing system comprising a photomultiplier tube, an amplifier circuit for amplifying the output signal, a peak height discriminating circuit for determining signal timing, a circuit for generating pulses of a specified time duration, and a coincidence circuit [K. Kuroda et al., Nucl. Instr. and Meth. A430(1999) 311-320]. In this method, in order to assure the desired minimum efficiency of simultaneous detection, the coincidence time which is set to perform simultaneous counting of signals corresponding to the horizontal and vertical directions has been chosen at specified values at least twice as much as the fluorescence life.
An object of the present invention is provide a two-dimensional radiation image detector that uses optical fibers such as wavelength shifting fibers in combination with scintillators or phosphors to detect signal for producing a two-dimensional radiation image and which can be easily fabricated by overcoming the difficulties encountered in the prior art in mounting a number of small scintillators in a plane.
Another object of the invention is to provide a two-dimensional radiation image detector that uses large pixels and which overcomes the difficulties encountered in applying phosphors such as a phosphor sheet.
Yet another object of the invention is to provide a two-dimensional image detector that uses the scintillator and the phosphor in combination as the radiation detecting medium to improve the detection efficiency by overcoming the difficulty in increasing the thickness of the detection medium.
A further object of the invention is to enhance the count rate of a two-dimensional radiation image detector that uses a scintillator or phosphor of short fluorescence life and which performs photon detection on the emitted fluorescence with optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a vertical and a horizontal direction.
A still further object of the invention is to provide a detector that enables fast and easy detection of two-dimensional images of not only a radiation but also neutrons.
These objects of the invention can be attained by a two-dimensional radiation image detector using the scintillator which is characterized in that grooves are cut in a scintillator sheet of large area in both a horizontal and a vertical direction at spacings of given pixel size and that a fluorescence reflector such as MgO or a material that not only reflects fluorescence but also has high gamma-ray absorbance or a material that not only reflects fluorescence but also has a large neutron absorption cross section is buried in the grooves to prevent inter-pixel leakage of fluorescence while improving the efficiency of fluorescence emission at the fluorescence detecting surfaces and enhance the position detecting performance for gamma-rays or neutrons through the use of the material having great ability to absorb gamma-rays or neutrons. By placing optical fibers such as wavelength shifting fibers within the grooves, fluorescence can be read from the lateral sides of the pixels and this enables the detection of multi-functional radiation image.
To fabricate a two-dimensional radiation image detector using the phosphor, transparent blocks or wavelength shifting blocks or scintillator blocks are used as fluorescence collecting substrates and optical fibers such as wavelength shifting fibers are placed on the lateral sides of the substrates for fluorescence detection as in the prior art.
By designing a structure in which the phosphor can be used in combination with the scintillator to make the radiation detecting medium, the detection efficiency can be improved even if it is difficult to increase the thickness of the phosphor.
In the case of a two-dimensional radiation image detector that uses a scintillator or phosphor having short fluorescence life and which detects the emitted fluorescence with optical fibers such as wavelength shifting fibers arranged in a grid pattern in both a horizontal and a vertical direction, a radiation image is constructed on the basis of the output photon detection signals for both horizontal and vertical directions. If pulse signals whose time duration is determined on the basis of the Poisson distribution in correspondence with the fluorescence life of the detection medium are generated from a retriggerable pulse signal generator that generates retriggerable pulses in response to a timing pulse signal output from a peak height discriminator, these pulse signals can be used to determine a two-dimensional radiation image in higher count rate by the simultaneous counting technique.
If desired, 6Li, 10B and Gd which are neutron converters that convert neutrons to ionizable radiation may be contained either alone or in admixture or in combination within the detection medium to make a neutron detecting medium which can be used to obtain two-dimensional neutron image.